Keeling´s statements on CO2 - data, and the detection of background level

• The mean value that Theodore de Saussure found was roughly 0.04% by volume, which I will put in modern units as 400 parts per million by volume (ppmv). This value was much less than von Humboldt had found, but still in considerable error.

• De Saussure's Memories, published in 1830, nevertheless ushered in a period of increasingly precise measurements of atmospheric carbon dioxide culminating in some nearly correct measurements in the 1880s by a Belgian named Jules Reiset.

• Unfortunately, none of the other investigators of carbon dioxide reproduced the seasonal cycle even approximately correctly, casting doubt on all of their data.

• After the 1880s, interest in carbon dioxide diminished for reasons which I have not been able to establish. Indeed, a quick and easy but not very precise 19th century technique of measurement, called Pettenkofer's method, became the most common method for measuring carbon dioxide, so that measurements actually became less precise than the best of those of the 19th century. The 20th century data were generally higher than the correct concentrations, although of course this wasn't known at the time.

• My first measurements were made with only a brief prior investigation of the scientific literature.

• In all cases I found the concentration of CO2 in the afternoon to be close to 315 ppmv. I came to the conclusion that the published 20th century data generally were in error and that the CO2 concentration, when not locally influenced, was nearly the same at least from the equator to the northern limits of the United States, perhaps everywhere.

Speech: Winner of the Second Blue Planet Prize (1993);
http://www.af-info.or.jp/eng/honor/bppcl_e/e1993keeling.txt